Just updated to iOS 6 but getting the iTunes error “There is no SIM card installed in the iPhone you are attempting to activate”? Here’s how to fix it.

This is a rather weird issue because you already have your SIM card inserted but iTunes is saying otherwise. What’s even more weird is that you have an iPhone 3GS which is already factory unlocked but won’t activate normally (or if you have an AT&T iPhone, and need to unlock it, click here).

If you have baseband 6.15.00 installed before, for whatever reason iTunes won’t let you activate your iPhone on iOS 6. Apple may have quietly put a this restriction in place. In case you missed the news, you can now downgrade from iPad baseband 6.15.00 to 5.13.04.

The SIM card inserted in this iPhone does not appear to be supported.
This iPhone is not currently setup to work with carrier you're attempting to use. Please insert another SIM card from a supported carrier or request that this iPhone be unlocked.

Tried this, waste of time
My iPhone 3GS was saying No Sim because the SIM tray connectors where Broken.
There should be six of those bronze little pins which should make contact with the SIM card when inserted.
Well three out of six of my pins where somehow missing, I guess they where broken when someone ripped the SIM tray holder out.
Try removing the SIM tray holder of your phone and shine a flash light inside the slot to see if there is any damage, even pocket dust. If you see something unusual then your problem lies there and the solution is to take your phone to a technician for repairs.